The present invention relates to a device for correcting of backlash, which arises on a change in the direction of movement in a drive mechanism and more particularly a power driven screw and a nut means co-operating with the screw, the nut means being connected to a unit which is movable back and forth along the screw. The device includes preferably a printing unit of a printer or a typewriter pulse generating means of the incremental type which in synchronism with the movement of the screw is arranged to generate two pulse trains having the same quadrangular shape but being 90.degree. out of phase. The pulse trains have a period corresponding to the displacement of the movable unit a predetermined distance along the screw and are arranged to be applied to a means which utilizes the pulse trains determine the position and the direction of the movable unit.
It is known in printers in connection with screw drive mechanisms of the kind referred to above to provide an incremental code disc that rotates with the screw and co-operates with a photoelectric device, in the usual way comprising light-emitting diodes and phototransistors, to generate two pulse trains being 90.degree. out of phase. From the two pulse trains information is derived about the position and the direction of movement of a printing unit movable back and forth along the screw.
In order for the movable unit to be easily moved along the screw a certain play or backlash must be present between the screw and a nut means co-operating with the screw. Usually, the backlash is compensated in a mechanical way, for example the nut means may comprise two nuts which are spring biassed away from each other and towards the flanks of the helical groove of the screw.